Optical components are becoming increasingly more common in telecommunication networks. For example, fiber waveguides such as optical fibers are used to carry information between different locations as optical signals. Such waveguides substantially confine the optical signals to propagation along a preferred path or paths. Similarly, other components such as sources, modulators, and converters often include guided regions that confine electromagnetic (EM) energy. Although metallic waveguides have a long history of use at longer wavelengths (e.g., microwaves), their usefulness as waveguides in the optical regime (e.g., 350 nm to 3 microns) is limited by their absorption. Thus, dielectric waveguiding regions are preferred in many optical applications.
The most prevalent type of fiber waveguide is an optical fiber, which utilizes index guiding to confine an optical signal to a preferred path. Such fibers include a core region extending along a waveguide axis and a cladding region surrounding the core about the waveguide axis and having a refractive index less than that of the core region. Because of the index-contrast, optical rays propagating substantially along the waveguide axis in the higher-index core can undergo total internal reflection (TIR) from the core-cladding interface. As a result, the optical fiber guides one or more modes of electromagnetic (EM) radiation to propagate in the core along the waveguide axis. The number of such guided modes increases with core diameter. Notably, the index-guiding mechanism precludes the presence of any cladding modes lying below the lowest-frequency guided mode for a given wavevector parallel to the waveguide axis. Almost all index-guided optical fibers in use commercially are silica-based in which one or both of the core and cladding are doped with impurities to produce the index contrast and generate the core-cladding interface. For example, commonly used silica optical fibers have indices of about 1.45 and index contrasts ranging from about 0.2% to 3% for wavelengths in the range of 1.5 μm, depending on the application.
Drawing a fiber from a preform is the most commonly used method for making fiber waveguides. A preform is a short rod (e.g., 10 to 20 inches long) having the precise form and composition of the desired fiber. The diameter of the preform, however, is much larger than the fiber diameter (e.g., 100's to 1000's of times larger). Typically, when drawing an optical fiber, the material composition of a preform includes a single glass having varying levels of one or more dopants provided in the preform core to increase the core's refractive index relative to the cladding refractive index. This ensures that the material forming the core and cladding are rheologically and chemically similar to be drawn, while still providing sufficient index contrast to support guided modes in the core. To form the fiber from the preform a furnace heats the preform to a temperature at which the glass viscosity is sufficiently low (e.g., less than 108 Poise) to draw fiber from the preform. Upon drawing, the preform necks down to a fiber that has the same cross-sectional composition and structure as the preform. The diameter of the fiber is determined by the specific rheological properties of the fiber and the rate at which it is drawn.
Preforms can be made using many techniques known to those skilled in the arts, including modified chemical vapor deposition (MCVD) and outside deposition (OVD). The MCVD process involves depositing layers of vaporized raw materials onto the inside walls of a pre-made tube in the form of soot. Each soot layer is fused shortly after depositing into a glass layer. This results in a preform tube that is subsequently collapsed into a solid rod, over jacketed, and then drawn into fiber.
The OVD process involves deposition of raw materials onto a rotating rod. This occurs in two steps: laydown and consolidation. During the laydown step, a soot preform is made from utlra-pure vapors of e.g., silicon tetrachloride (for silica fiber) and germanium tetrachloride. The vapors move through a traversing burner and react in the flame to form soot particles of silicon oxide and germanium oxide. These particles are deposited on the surface of the rotating target rod. When deposition is complete, the rod is removed, and the deposited material is placed in a consolidation furnace. Water vapor is removed, and the preform is collapsed to become a dense, transparent glass.
Another method for making a fiber preform is to simply insert a rod of one material into the core of a hollow preform. Heating consolidates the preform into a single object.
Fiber waveguides form the basis of numerous optical devices in addition to simply providing a channel for the transmission of optical information. For example, fiber waveguides can be design to compensate for effects that may be deleterious to an optical signal, e.g., dispersion. Dispersion is the property of a waveguide that causes optical signals of different wavelengths to travel at different speeds, which results in broadening of optical pulses. Typically, a long haul silica optical fiber has a positive dispersion of 2–50 ps/nm-km for wavelengths in the range of 1.5 μm. This positive dispersion can be compensated by directing the signal through a waveguide having negative dispersion equal in magnitude to the positive dispersion introduced by the silica optical fiber. Often, this is implemented by providing alternating sections of fiber having positive and negative dispersion in an optical telecommunications network.
Another example of an effect that may be deleterious to an optical signal is attenuation. Attenuation is simply the loss of intensity of an optical signal that occurs as a signal propagates through an optical fiber. When attenuation is sufficiently large, the optical signal becomes indistinguishable from the background noise. Accordingly, important components in communications networks are fiber amplifiers. As their name implies, fiber amplifiers are fiber waveguides that amplify the signal strength of an optical signal. The growth of dense wavelength-division multiplexing applications, for example, has made erbium-doped fiber amplifiers (EDFA's) an essential building block in modern telecommunication systems. EDFA's amplify an optical signal inside a fiber and therefore allow transmission of information over longer distances without the need for conventional repeaters. To form an EDFA, the fiber is doped with erbium, a rare earth element, that has appropriate energy levels in its atomic structure to amplify light at 1550 nm. A 980 nm pump laser is used to inject energy into the erbium-doped fiber. When a weak signal at 1550 nm enters the fiber, the light stimulates the erbium atoms to release their stored energy as additional 1550 nm light. This stimulated emission is coherent with the original signal, and hence the original signal grows stronger in intensity as it propagates down the fiber.
A fiber laser is another example of an optical component made using optical fibers. Typically, the cavity is defined in the radial direction by the index difference between a high index core and a lower index cladding which confines EM radiation through total internal reflection (TIR). The cavity may be defined in the axial direction by reflectors. The end reflectors in early fiber lasers were mirrors placed at, or evaporated onto, the ends of polished fibers. Refractive index modulations along the fiber axis can also be used to create a reflector and thus define a lasing cavity. For example, two Bragg gratings can surround a gain medium and define the end reflectors, thereby forming a distributed Bragg reflector (DBR) laser. Alternatively, the axial modulation can extend through out the length of the gain medium to form a “distributed feedback” (DFB) laser.
The composition of typical fiber waveguides often consists of a single material, having an appropriately doped cross-sectional profile to manipulate the fiber's optical properties. However, compositions including different materials may also be used. Accordingly, compositions including dissimilar materials, fiber waveguides derived from the dissimilar material compositions, and exemplary devices are disclosed.